Heterogeneous reactions are widely employed in both industry applications and lab scale experiments. Gas-solid, liquid-solid, and gas-liquid reactions are the most common examples of heterogeneous reactions. The physical properties of solid surface, liquid streams and gas streams change as the reaction(s) proceed. If one of those physical properties can be measured as a function of time, then the degree of saturation or available capacity can be estimated. Such estimates can be used to determine when the solid surface needs to be replaced in order to optimize the efficiency and efficacy of the heterogeneous reaction.
Significant research and development efforts have been made to develop a means for monitoring adsorbent capacity, especially for desulfurization systems for fuel cell applications. The majority of these efforts involve measuring changes in the sulfur concentration in a fuel stream upon passage through a sorbent bed. For example, U.S. Patent Application Publication No. 2008/0038603 to Lee et al. describes a method to determine sulfur concentration by measuring changes in electrical, physical, or chemical properties during desulfurization. The methods described in Lee require at least two sensors, one at the inlet of the sorbent bed and one at the outlet of the sorbent bed, which are used to measure the difference in sulfur concentration upon passage through the sorbent bed. The difference in sulfur concentration is then used to estimate the degree of saturation of the adsorbent material
U.S. Pat. No. 6,716,336 to Hurland describes a sulfur sensor to determine sulfur concentration of a liquid stream. The methods involve measuring the change in potential between a working electrode and a reference electrode separated by a silver ion conductor. Sulfur bonds readily with silver ions, therefore, the potential changes with sulfur concentration in the stream.
Both of the methods described above use sulfur concentration as a means for estimating the remaining adsorptive capacity of an adsorbent material. In the case of Hurland, streams containing low sulfur concentrations may produce potential changes that are undetectable by the method described therein.
Changes in optical properties have been also been used for catalyst characterization to determine adsorbate concentration. For example, commercial in-line H2S detectors can measure H2S concentration by measuring the color changes of lead acid paper in process stream containing H2S. Although it has stringent requirements for relative humidity, temperature and exposure time for accurate measurement, this method provides an economical way to measure sulfur concentration. In order to continuously measure H2S concentration, a roll of lead-acid tape is continuously fed to the detecting unit which increases cost of the this method and requires disposal of the lead-acid tape.
U.S. Pat. No. 6,755,015 to Manaka describes methods for measuring NOx adsorption. For example, Manaka describes measuring chemical species concentration before and after the adsorbent bed. If the concentration or signal captured downstream of the bed is higher than a critical value, then the adsorbent bed is considered exhausted. Alternatively, NOx removal rate can be determined based on concentration or signal captured both upstream and downstream of the adsorbent bed. If the NOx removal rate is blow a critical value, then the bed is also considered exhausted.
None of the prior art discussed above directly measures the degree of saturation of the adsorbent material or catalyst bed itself; rather, the prior art methods involve measuring sulfur concentration and using that measurement to estimate the remaining adsorptive or catalytic capacity. While these approaches may be effective with a constant adsorbate concentration or known adsorbent concentration profile in the gas stream, these methods may have limitations with significant changes in concentration, i.e., high concentration variations. For example, if the adsorbate (sulfur or NOx) concentration suddenly increases, the outlet concentration of the chemical species may increase to above the critical breakthrough concentration. According to Manaka's method, capacity depletion will be detected. Similarly, if the inlet concentration has a sudden drop in its concentration, the downstream concentration and calculated adsorbate removal rate will drop too and another capacity depletion signal will be given. In order to determine false signals, the relationship of the adsorbate concentration and degree of saturation must be established for various scenarios. Moreover, the methods described above require at least two sensors, at the inlet and outlet or before and after the bed, in order calculate sulfur or NOx concentration. Systems with two sensors are more prone to reliability issues than a single sensor system.
In view of the limitations discussed above, there exists a need for a method of determining adsorptive capacity or catalyst deactivation that does not require two sensors and the accuracy of which is not dependent on measuring the concentration of one or more contaminant species.
Therefore, it is an object of the invention to provide methods for determining adsorptive capacity or catalyst deactivation that does not require two sensors.
It is also an object of the invention to provide methods for determining adsorptive capacity or catalyst deactivation the accuracy of which is independent of the concentration of one or more contaminant species.